1. Field of the Invention
The invention relates to a group III nitride semiconductor device (also simply referred to as a device, hereinafter) and, more particularly, to a method of producing the device.
2. Description of the Related Art
In the field of light emitting devices such as light emitting diodes, semiconductor laser diodes or the like, a semiconductor light emitting device having a crystal layer obtained by adding a group II element such as magnesium (Mg), zinc (Zn) or the like into a single crystalline group III nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) attracts much attention as a device which can emit a blue light.
Epitaxial growth of nitride semiconductor is generally performed by a metalorganic chemical vapor deposition (MOCVD) method. An as-grown layer of a group III nitride semiconductor crystal (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) element such as Mg, Zn has been added by using MOCVD, however, usually exhibits extremely high resistivity. Attempts to achieve a blue light emitter using this material have been hampered by this inability of p-type conduction.
In recent years, it has been reported that low-resistivity p-type could be obtained by performing a special treatment to the high-resistivity (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) doped with group II element such as Mg. H. Amano et al. found out that low-resistivity p-type conduction was realized by performing a low-energy electron-beam irradiation to the crystal (H. Amanoetal. :Jpn. J. Appl. Phys. Vol.28, 1989, pp.L2112-2114). S. Nakamura et al. has found out that a low-resistivity p-type crystal can be also realized by performing a heat treatment to the crystal at a temperature in a range from approximately 700 to 800xc2x0 C. in nitrogen ambient under atmospheric pressure or under high pressure (S. Nakamura et al.: Jpn. J. Appl. Phys. Vol.31, 1992, pp. L139-142).
The mechanism of the treatment for establishing those p-type conduction is interpreted in such a manner that, the hydrogen atoms which passivate the group II acceptor impurities such as Mg or the like by combining with them in the grown film are dissociated by the above treatments.
According to the above method of the low-energy electron-beam irradiation, an extremely high room temperature hole concentration on the order of E18/cc is obtained in a resultant p-type crystal. However, a treated depth is limited within a penetration depth of the electron beam and, for example, the treated depth is equal to about 0.3 xcexcm with an accelerating voltage of 6 to 30 kV (S. Nakamura et al.: Jpn. J. Appl. Phys. Vol.31. 1992, pp. L139-142). Since the low-energy electron-beam irradiation treatment is performed by scanning the wafer surface with an electron beam in a vacuum vessel, the equipment is apt to be bulky and moreover a long treatment time is needed, so that this method is unfavorable for the mass production of the laser devices.
On the other hand, the above method of the heat treatment is free from the strict limitation of the treated depth as in the case of the low-energy electron-beam irradiation process. It is considered that the heat treatment is suitable for mass production since a number of wafers can be treated in a batch by a heating furnace.
In the case of producing a semiconductor laser device by using the heat treatment as mentioned above, a contact resistance at an electrode of the device remains as a problem, since the room-temperature hole concentration adjacent to the electrode is equal to approximately 3E17/cc. If a treatment temperature is raised in an attempt to increase the hole concentration of the film as a whole, electric characteristics of the device are contrarily deteriorated. It is thought that this is because nitrogen vacancies are generated near the film surface due to nitrogen dissociation, the nitrogen vacancies act as donors and compensate the acceptors, and the hole concentration in a region near the surface contrarily decreases. A contact characteristic of the electrode is, consequently, deteriorated.
Several countermeasures against the nitrogen dissociation problem as mentioned above are conceivable.
For example, as a first countermeasure, there is a method of extremely raising the pressure of nitrogen ambient for the heat treatment. It has been found that if the heat treatment is performed under a high nitrogen pressure of approximately 90 atm, no surface degradation occurs even at a temperature of approximately 1000xc2x0 C. (S. Nakamura et al.: Jpn. J. A.ppl. Phys. Vol.31, 1992, pp. L1258-1266).
In the case of using the above method, however, since an extremely high pressure and temperature used in the process requires a special chamber for the heat treatment, resulting in an obstacle to the realization of mass production.
There is a second countermeasure against the nitrogen dissociation problem in which a protecting film or cap layer made of another material is formed on a crystal layer of the group III nitride semiconductor doped with the group II impurity element and, thereafter, the heat treatment is performed. The material of the protecting film is required to have hydrogen permeability so as not to obstruct the elimination of hydrogen while preventing the dissociation of nitrogen from the crystal. Silicon dioxide (SiO2), silicon nitride (Si3N4), aluminum nitride (AlN) and the like can be mentioned as candidate materials which can endure the temperature condition to be used for the heat treatment as mentioned above.
The protecting film SiO2 is stable at a comparative high temperature and can be easily removed by wet etching using hydrofluoric acid (HF) or the like. Since GaN is hardly eroded by HF, the protecting film SiO2 formed on a GaN layer has such an advantage that the HF etching is automatically stopped at a time when the proper amount of film is etched. There are, however, the following drawbacks.
An SiO2 film is formed by a sputtering method or the like, o (oxygen) deficiency fends to occur during the film formation to become SiOx (where subscripted x denotes an atom ratio), so that sputtering has to be performed while adding an oxygen gas. In this instance, oxygen penetrates into the nitride semiconductor film. Since oxygen acts as a donor in the group III nitride semiconductor to compensates the acceptor in the crystal of the group III nitride semiconductor doped with group II impurity, the hole concentration in a region near the surface of the group III nitride semiconductor is contrarily decreased. As a result, an expected effect cannot be derived.
A cap layer of Si3N4 also has sufficient high-temperature stability, however, due to its extremely high chemical stability, it is difficult to be removed by chemical etching, An RIE (reactive ion etching) therefore, has to be used for removing the Si3N4 film. In removal of the cap layer, etch selectivity mentioned above becomes a serious requirement. The ideal selectivity is that obtained for the etching of SiO2 layer on GaN layer by HF.
The thickness of each constituent layer of a device is of great importance, especially for laser diode. The uppermost layer, namely, a contact layer (Mg doped GaN layer in this case) for an electrode of the device is formed to have a relatively small thickness of 0.1 xcexcm. Since high selectivity cannot in RIE, it is extremely difficult to remove the Si3N4 cap layer completely while leasing the contact layer with a predetermined thickness.
To realize a practical laser device, it is necessary to form some refractive index waveguide structure in the device. The waveguide structure most generally used is what is called a ridge structure. Although a ridge structure can be formed by removing some portions of the semiconductor crystal layer (while) leaving a thin ridge portion, a precise control is required on the removal. Since the ridge forming step is performed after the cap layer removing step, the error caused in the cap layer removing step is directly reflected to an error of a ridge height.
There arises further inconvenience that, since RIE utilizes a plasma unlike pure by chemical etching, the crystal surface was to be irradiated with high energy particles, so that crystal defects are formed on the surface. Since the existence of those defects deteriorates the contact of electrode, an expected effect cannot be derived also in this case.
Although AlN is a group III nitride semiconductor similar to GaN, it has much better high-temperature stability than that of GaN and hardly dissociates at a heat treatment temperature within a range from 800 to 1000xc2x0 C. In the case of using AlN as a material for the protecting film, the film may be formed by a reactive sputtering using an aluminum target and nitrogen gas (N2). In this case, it is considered that no oxygen penetration occurs during the formation of the protecting film.
Since AlN can be easily chemically etched in a heated potassium hydroxide (KOH) aqueous solution and GaN (at least monocrystalline GaN which is used here) is never eroded under this condition, the etching automatically stops at the interface between them, so that it is very convenient.
An experiment reveals, however, that sufficient effect, namely, an enough improvement of the p-side electrode characteristics cannot be derived. It is considered that this is because the hydrogen permeability of AlN is insufficient.
The third countermeasure method against the nitrogen dissociation problem is as follows: A crystal layer of an n-type group III nitride semiconductor is subsequently formed on the crystal layer of the group III nitride semiconductor doped with the group II impurity element and, after the completion of the growth, the crystal layer of the n-type group III nitride semiconductor is removed.
The above method is based on the findings that most of the hydrogen atoms which passivate the acceptors in the crystal of group III nitride semiconductor doped with group II impurity are those diffused into the crystal through the surface during the cooling step after the growth, and that hydrogen atoms hardly penetrate an n-type semiconductor layer. While this procedure is advantageous in that it does not require any post-growth heat treatment. The n-type layer tends to be thick(in oder)to obtain a sufficient p-type conductivity in the underlying Mg-doped film.
The third method also has the drawbacks mentioned with respect to the second method. That is, the dry etching such as RIE or the like is necessary to remove the n-type layer, there is no selectivity in dry etching between the cap layer (i.e., n-type GaN layer here) to be removed and the underlying layers (i.e., Mg doped GaN layer here).
Since AlN can be chemically etched with KOH aqueous solution, as mentioned above in the description of AlN protecting layer, if the cap surface layer is made of AlGaN having a high AlN composition ratio ( greater than 50%), chemical etching can be applied with a high selectivity. However, it is known that AlGaN in this composition range becomes an insulating material irrespective of the addition of impurities. This is because a general donor impurity such as Si, O, or the like does not act as an effective donor in this composition range as is theoretically supported. The diffusion and penetration of hydrogen atoms, therefore, cannot be prevented when using AlGaN of this composition range.
As explained above, there has been no method of producing the group III nitride semiconductor device having a high hole concentration in the p-type GaN film without degrading the electrode characteristics, process accuracy, and productivity when applied to laser devices.
It is, therefore, an object of the invention to provide a group III nitride semiconductor device producing method which can improve a hole concentration and obtain an sufficient effect without causing a burden on a device producing step in order to realize a crystal layer of low-resistivity p-type.
According to the invention, there is provided a method of fabricating a group III nitride semiconductor device, comprising the steps of:
forming a first crystal layer made of a group III nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) doped with a group II impurity element;
forming a second crystal layer made of a second group III nitride semiconductor AlzGa1xe2x88x92zN (0.7xe2x89xa6zxe2x89xa61) onto the first crystal layer; and
removing at least a part of the second crystal layer by etching after the formation of the first and second crystal layers.
In an aspect of the fabrication method according to the invention, the step of forming the second crystal layer is started at substantially the same temperature as a forming temperature of the first crystal layer.
In another aspect of the fabrication method according to the invention, the first and second crystal layers are formed by a metalorganic compound chemical vapor deposition method.
In a further aspect of the fabrication method according to the invention, a nitrogen precursor in the metalorganic chemical vapor deposition method is ammonia.
As to a still further aspect of the fabrication method according to the invention, the amount of ammonia supplied into the reactor vessel is set much lower during the growth of the second crystal layer and the subsequent cooling stop or during the cooling step alone than that for the growth of the first crystal layer.
In another aspect of the fabrication method according to the invention, at the end of the growth of the second crystal layer, the shut off of (or the feed of) group III precursor.
In a further aspect of the fabrication method according to the invention, at the end of formation of the second crystal layer, supply of group III precursor is terminated and cooling is started at the when the indication of ammonia detector attached to the exhaust line of the reactor falls below a predetermined value.
In a still further aspect of the fabrication method according to the invention, the step of formation of the second crystal layer includes a usage of nitrogen gas as a carrier gas.
According to the method of producing group III nitride semiconductor device of the invention, AlN film formed is used as a cap layer, i.e., a second crystal layer onto the group III nitride semiconductor layer doped with group II impurity, i.e., a first crystal layer, amount of ammonia (NH3) which is introduced into a reactor vessel is exceptionally reduced when the AlN layer is grown. Moreover, the AlN cap layer is removed by chemical etching with a sufficient selectivity to underlying GaN, so that a device wafer having the first crystal layer whose surface has a high hole concentration can be obtained without any post-growth heat treatment.